Many conventional position detecting devices detect a rotation angle of a throttle valve, which controls a quantity of air sucked into a cylinder of an internal combustion engine for a vehicle. The position detecting device has a magnetic detection element for detecting a magnetism developed by a magnet disposed in the throttle valve and outputs the detected magnetism value to an electronic control unit (ECU) for the vehicle. The ECU supplies a current to a motor for rotating the throttle valve so that the rotation angle of the throttle valve, which is calculated on the basis of a signal output from the position detecting device matches a target rotation angle of the throttle valve, which is set variably according to an operating state of the internal combustion engine.
The position detecting device disclosed in JP 3588127 (U.S. Pat. No. 5,260,877) includes two integrated circuits. One integrated circuit has a positive characteristic, by which a signal to be output to the ECU increases as the magnetism that acts on the magnetic detection element increases. The other integrated circuit has a negative characteristic, by which the signal to be output to the ECU decreases as the magnetism that acts on the magnetic detection element increases.
In JP 3588127, an abnormality of the position detection device is detected based on an assumption that a sum of signal values output from the two integrated circuits is theoretically kept constant.
However, when output terminals of the two integrated circuits are short-circuited, an intermediate potential of voltage signals to be output from the two integrated circuits is output to the ECU. Even in this case, the sum of the signal values output from the two integrated circuits is generally kept constant. For this reason, the position detecting device of JP 3588127 cannot detect short-circuiting of the output terminals of the two integrated circuits. For this reason, when the output terminals of the two integrated circuits are short-circuited, and a larger current flows from one integrated circuit into the other integrated circuit than in a normal state, output circuits provided in the integrated circuits are likely to be damaged.